Summary of strategies to reduce N losses
to water from agricultural systems
Summary in other languages:
The NTOOLBOX project began in 2009 as a European Union sponsored project to identify practical, on-farm strategies to reduce losses of nitrogen to water. Throughout the project we have produced a range of documents that outline the state-of-the-art in the science and practice of implementing N loss reducing strategies (NLRS) at the farm level. If you explore these web pages you will find many of these documents in pdf format or converted into web pages. On this page, we provide a summary of the key strategies that we have identified.
The NLRS have been divided into the following 8 categories:
- Manure storage and handling solutions
- Livestock management
- Pasture management for reduced N losses
- Balanced N application rates
- BMP for manure use on land
- Strategies for irrigated land
- Efficient N cycling at the field level
- Runoff, drainage and wastewater management
1. Manure storage and handling solutions
On any farm where livestock are raised and kept in confinement for some period of time (e.g. not in extensive, pasture-based systems) manure needs to be collected and stored before application to the land. Problems arise when there is insufficient storage capacity for the manure and farmers are forced to spread manure at inappropriate times. This is especially a problem in northern regions where there is a pollution risk when spreading manure during the winter on frozen ground and/or when crops are not actively growing. For this reason, enhancing manure storage capacity is advisable. Composting manure or bedding livestock on high C materials such as straw can help to conserve nitrogen by enhancing the conversion of available N into more stable organic N. Risk of contamination of waterways by runoff from manure piles can be reduced by siting solid manure heaps away from watercourses and field drains. Covering solid manure storage areas reduces runoff, thereby reducing the volume of dirty water that must be collected and stored before application to the land. It also improves the quality of the manure since valuable nutrients, especially potassium, are conserved within the solid manure. We also recommend storing solid manure on a concrete pad with a runoff collection system; this will prevent the transport of NO3- in runoff and leachate. Equipment is now available to separate solid and liquid fractions of manure so that each component can be used in a targeted way, e.g. the solid component can be used as a soil improver, organic fertilizer or as a component of a compost-based plant-growth medium. The liquid fraction contains low concentrations of nutrients in a readily-available form and can be used in a similar way to mineral fertilizer, or in fertigation systems. Finally, turn the manure "problem" on farm, into a resource which can be used for energy generation by using anaerobic digestion of slurry. The process also concentrates the nutrients in the original material making it less energy intensive to move and spread the digestate. This is an ideal approach for indirectly addressing the problem of N losses to water from livestock operations, particularly intensive livestock operations (ILOs).
2. Livestock management
Livestock management NLRS are focussed on improving efficiency of feed use so that quantities of nutrients excreted by stock are minimised. On dairy farms one approach is to decrease the number of young cattle and reduce the cattle turnover rate. This is effective because in older cows, a higher proportion of the dietary protein is used for milk protein and less is used for maintenance; therefore, the N excretion per kg milk is lower in older cows. Another strategy is to reduce the levels of dietary N in the feed. By avoiding excess N in feed as much as possible, the amount of N unused by the animal is reduced and so also the amount of N excreted. Providing diets that are nutritionally optimized for a specific growth or reproductive stage or, in the case of dairy cattle, lactation cycle, is known as phase feeding. This allows for more complete use of the nutrients in the feed for animal production, and less excretion of excess nutrients in the manure. In ruminants balancing dietary nitrogen and carbohydrates to optimize rumen function can also lead to more efficient use of feed.
3. Pasture management for reduced N losses
Pastures can be a "hot-spot" for N leaching on farms with grazing animals. Pasture management strategies focus on eliminating high risk areas (e.g. around feeders, gates) and facilitating more even movement of animals around the pasture and more even distribution of urine and faeces at the same time. "Poaching" is the damage caused to soils due to excessive foot traffic when they are wet. Soils can become puddled causing reductions in water infiltration and increased runoff. Reduced damage to soil structure by foot traffic can be achieved by moving feed and water troughs regularly, placing feeders and troughs on hard bases, re-siting gateways away from high risk areas, and reducing stocking densities when soils are wet. Excluding livestock from surface waters is also recommended by fencing waterways, providing alternative drinking systems and constructing bridges for livestock crossing streams. Another strategy to reduce N losses from pastures is to increase the clover content of the sward. This reduces the need for N fertilisation; nitrate leaching is generally lower from unfertilised grass-clover swards than from mineral-N fertilised swards grazed by either sheep or cattle. There is evidence to indicate that older, permanent swards have higher rates of N losses due to leaching than more recently seeded swards. Therefore it is advisable to reseed older, permanent swards. In addition, reducing stocking densities on pastures allows an overall reduction in the amounts of urine and faeces deposited on pasture land. Finally, nitrification inhibitors have been successfully used on pastures in New Zealand to reduce nitrate leaching, although this technology is not yet commercially available in Europe. These inhibitors slow down the biological conversion of ammonium-N to nitrate-N thereby reducing the risk of leaching.
4. Balanced N application rates
Applying only enough N as manure or fertiliser is another way to reduce risks of N leaching. Optimum fertiliser rates can be determined by using local fertiliser recommendation handbooks and software packages, and soil testing prior to fertiliser application. Once the optimum fertiliser rate is decided, these rates can be fine tuned by using in season estimates of crop N status. There are a number of tools on the market including hand held chlorophyll meters, tractor-mounted sensors, and satellite imagery services.
5. Best management practices for manure use on land
Improving manure storage and handling can reduce losses of N to water, but the greatest risks arise during and after manure is applied in the field. This risk can be reduced if you do not apply manure to high-risk areas e.g. areas adjacent to watercourses, shallow soils over fissured rock or cracked soils over field drains, areas with a dense network of surface drains, or wet depressions draining to a nearby watercourse. In addition it is advisable to apply manure to land when conditions are optimum, i.e. when there is a low risk of nutrient runoff or leaching. In many regions there is legislation that prohibits spreading during high risk periods, e.g. when ground is frozen. Manure testing should be used to understand its available N content before it is spread in the field. Samples can be sent to the laboratory, or an on-farm test kit can be used. In addition to understanding the available N content of manure, it is important to accurately estimate manure N release over the growing season using manure reference tables or software packages. Finally, regular maintenance and calibration of manure application equipment will ensure even and accurate application.
6. Strategies for irrigated land
Irrigated systems are a special case where leaching of nitrates is closely linked to water management. In irrigated systems, in addition to following the recommended practices already listed, farmers should adjust the quantity of water applied to match crop needs. This requires proper irrigation scheduling according to crop evapotranspiration and application of irrigation water in smaller amounts more frequently. Fertigation (the supply of crop nutrients directly with the irrigation water) allows precise and frequent application of nutrients to coincide with crop needs. The result is lower total application rates of fertilizers which reduces costs and reduces the risk of N leaching. Finally, upgrading the existing irrigation to a more water use efficient system will result in less movement of nitrates below the root zone during crop growth. However, it is important that no surplus nitrate remains in the soil profile after crop harvest, as this will be susceptible to leaching during the winter rains. Strategies to optimize fertiliser use during crop growth and maximise efficient N cycling in the field should be used in irrigated systems to ensure that leaching of nitrates is not simply postponed to the winter season.
7. Efficient N cycling at the field level
Closely linked to balanced N application rates are strategies to ensure the most efficient use and cycling of N in the field. Splitting fertiliser or manure applications can ensure that N supply more closely coincides with periods of crop demand. Since soil disturbance (e.g. tillage) stimulates the release of N from organic matter and crop residues, it is recommended to cultivate land for crop establishment in spring rather than autumn. This will ensure that a crop is growing and able to take up newly released N soon after tillage. Catch crops are cover crops grown to catch available N in the soil and thereby prevent N leaching losses. Farmers are advised to use a catch crop when possible, but especially following a crop that leaves high levels of residual nitrate in the soil after harvest. Another alternative is to incorporate residues with a high C:N ratio into the soil to promote immobilisation of mineral N and reduce the risk of leaching. Straw or other high C:N ratio residues can act as a "sponge" that holds inorganic N in the soil during the season when no crop growth is occurring. The use of N fixing green manure crops in the rotation can reduce reliance on artificial N fertilizers. When N fixing green manures (like crimson clover, vetch, alfalfa) are ploughed into the soil the plant N becomes part of the soil organic N pool which is gradually released to future crops. This process is less likely to lead to the build up of nitrates in the soil solution and therefore reduces the risk of N leaching.
Slow- or controlled-release fertilisers have a protective coating or encapsulation which controls water entry and rate of dissolution, and/or may contain a nitrification inhibitor. The release of nutrients from these fertilisers is more synchronized with plant uptake, and losses from the soil by denitrification or leaching are reduced.
Efficiency of N cycling in the field can be further improved by using a crop rotation in which N efficient and inefficient crops are rotated. The overall goal of this strategy is to diversify the crop rotation so that crops that are shallow rooted and leave behind relatively high levels of nitrate in the soil, are alternated with deep rooted crops that can capture residual nitrate from deep in the soil profile and prevent it from leaching below the root zone.
8. Runoff, drainage and wastewater management
Runoff, drainage and wastewater management strategies can be called "end of pipe" solutions to the problem of nitrate pollution. When all the strategies already listed in this catalogue are in place, there is still some risk of N pollution of water from agricultural activities. This is especially the case in areas where there are periods of intense, heavy rainfall which can result in large volumes of dirty water running off farmyards and fields.
To reduce the risk of N pollution in these situations, it is advisable to improve yardworks for clean and dirty water separation. This reduces the risk of dirty water being directed into local watercourses and also reduces the size of the dirty water storage tank needed on the farm. There is a risk of runoff of sediment and nutrients from fields that have been recently tilled if there is a heavy rainfall. Sedimentation ponds can be installed to collect surface runoff from fields and allow sediments to settle out, while aquatic plants growing in the pond can take up excess nutrients; the water that flows out of the pond when it is full is therefore relatively clean. Similarly, artificial wetlands can be constructed to treat dirty water that is collected from farm yard runoff. Along the edges of streams and wetlands riparian buffer strips can be planted. These strips of grass and/or trees slow down the rate of water runoff from the field, allowing the water to infiltrate the soil (entering subsurface flow) where it is more susceptible to uptake by plants and denitrification by soil micoorganisms.